西南日本における降水の継続性からみた東アジア梅雨季の総観場

Abstract

In this study, the rainfall distribution and synoptic conditions concerning behavior of the frontal zones over East Asia during the Baiu season have been investigated in terms of the length of consecutive heavy rainfall duration in the southwestern part (between 130°E and 140°E) of the Japanese Islands. In the cases of consecutive rainfall which lasts several days in the southwestern part of Japan, the Subtropical Frontal Zone (STFZ), which corresponds to a convective cloud zone extending from the inland region of China, causes rainfall in Japan as the Baiu Frontal Zone (BFZ). The Eurasian Polar Frontal Zone (EPFZ) is located along the 50°N latitude line as a maximum zone of meridional temperature gradient. On the other hand, when rainfall lasts a day or two, the EPFZ is located between 35°N and 40°N to the east of 120°E, and rainfall in the southwestern part of Japan is caused by a sub-synoptic-scale cyclone moving along the EPFZ. In this case, the STFZ is inactive and it comes very close to or merges with the EPFZ. The structure of the BFZ of this case is considered to be different from that of the STFZ but comparable with that of the EPFZ. Moisture content and moisture flux around Japan during the heavy rainfall period are larger in the former case than in the latter case. Moreover, preceding the start of the rainfall of longer duration in the southwestern part of Japan, an area of active convective clouds appears in the inland region of China.